Effect of the Parameters of Semi-Solid Processing on the Elimination of Sharp-Edged Primary Chromium Carbides from Tool Steel

Abstract

Although conventional tool steels have been heat treated on a routine basis for decades, the search continues for new ways to eliminate their troublesome sharp-edged primary chromium carbides, which impair toughness. One of the available techniques is semi-solid processing, which involves partial melting of the workpiece. The structure after semi-solid processing consisted of a austenite and carbide-austenite network. The network can be broken up and its fragments distributed uniformly by subsequent forming with appropriate parameters. In this experimental study, X210Cr12 tool steel was heated to a semi-solid state, and after cooling to a solid state, worked in a hydraulic press. Suitable soaking temperatures were sought within an interval between 1200 °C and 1280 °C. The workpieces were quenched from the forming temperature in water or oil. In order to improve formability and reduce hardness, tempering was tested as well. Additional experimental regimes included conventional quenching and tempering. Once the appropriate parameters were chosen, the elimination of primary chromium carbides was successful. The resultant microstructures were fine and consisted of M-A constituent with a size of approximately 1 μm, and very fine Fe3C and Cr7C3 carbides. The hardness was in excess of 800 HV10. They were examined using optical and scanning electron microscopes. The carbides were characterized on transparent foils in a transmission electron microscope. Mechanical characteristics were determined in micro-tensile tests.